What kind of NIC does the guest see? I've only had luck with the Intel PRO/1000 MT Desktop one, although I don't remember the specific problems I had with the other types. I'm also using a Marvell Yukon 88E8056 PCI-E adapter on a Window 7 x64 host, and a variety of Linux/Solaris/*BSD guests.
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Scott McClungMar 21 '12 at 6:16